1. Field of the Invention
The present invention relates to a process for producing a thin die-cast molded article of an aluminum material. More particularly, the present invention relates to a process for producing a thin die-cast molded article of an aluminum material having a thickness of 0.4 to 1.2 mm and exhibiting an excellent appearance using a mold having a narrow gate efficiently with excellent flow of the melted metal through the gate portion.
2. Description of Related Art
Aluminum has excellent properties as a metal such as a light weight, an excellent plastic working property, excellent corrosion resistance and high electric and thermal conductivities. It is known that alloys obtained by adding copper, magnesium, zinc, silicon, lithium, nickel, chromium, manganese, iron or zirconium exhibit remarkably improved mechanical properties at the ordinary temperature and high temperatures due to hardening by formation of solid solutions, work hardening and age hardening and acquire advantageous properties such as corrosion resistance, wear resistance and low coefficients of thermal expansion. Therefore, aluminum and aluminum alloys having the advantageous properties are widely used in many fields, for example, in the fields of utensils such as cans of drinks, furnitures and interior articles; aviation and space; automobiles; electric and electronic products; vehicles; ships; and civil engineering and buildings.
As one of the processes for working aluminum materials such as aluminum and aluminum alloys, the die-cast process is well known and widely used for producing various molded articles.
The die-cast process is a casting process in which a melted material is injected into a mold made of a metal by an injecting plunger at a high speed (about 20 to 60 m/second) under a high pressure (about 30 to 200 MPa), fills the mold and solidifies rapidly. The die-cast process is advantageous in that thin cast products having a minimum thickness of about 1 mm can be produced, products having an excellent surface can be obtained with excellent accuracy of dimensions and the productivity is high.
However, since the die-cast process is conducted by injecting a melted metal at a high speed and the time of filling is as short as 0.3 seconds, the die-cast process has a drawback in that the air in the space (the cavity) of the mold and gases formed by the reactions tend to be sucked into the product and defects of pores (porosity) tend to be formed. Therefore, various processes have been developed for improving the quality. Examples of such processes include (1) the low speed filling die-cast process in which the speed of injection is lowered to 1 m/second or slower; (2) the ACURADE process in which the pressure is added in two steps to prevent formation of sink in portions having a greater thickness; (3) the local SQUEEZE process in which portions having a greater thickness are locally pressed to prevent formation of sink; (4) the vacuum die-cast process in which the pressure at the inside of the mold is lowered to decrease sucking of gases; and (5) the PF die-cast process in which the inside of the mold is filled with oxygen and sucking of gases is prevented by dispersing the entire oxygen as fine particles of oxides.
However, the above conventional technology has been developed not for producing thin products but for improving the quality of the die-cast products. The maximum thickness of the die-cast molded article of an aluminum material is about 1 mm. It is the actual present situation that no die-cast molded articles having a thickness smaller than 1 mm have been obtained.
Recently, devices having many semiconductors and IC's equipped with ultra-small electronic circuits in which inner wirings are combined together in a solid article in accordance with specific methods are increasing. The above devices and IC's have the possibility of troubles that the working of the semiconductors becomes unstable and the semiconductors may be broken due to a high temperature caused by a great amount of heat generated in the process of operation of the semiconductors. The unstable working and the fracture of the semiconductors due to the high temperature are prevented by attaching a heat radiating plate for cooling the semiconductors so that the heat of the semiconductors are released into the air by heat exchange between the heat radiating plate and the air.
As the heat radiating plate, in general, a substrate attached with numerous thin and long heat radiating pins is used. Die-cast molded articles of aluminum materials are frequently used for the above heat radiating plate. As IC is made still more compact recently, a decrease in the weight of the above heat radiating plate is desired.
For case covers of portable electronic instruments, die-cast molded articles of aluminum materials are widely used. As the size of the electronic instruments is decreasing, for example, as personal computers of the note type are more widely used, a decrease in the weight of the case cover is desired.
As the material having a light weight, magnesium materials such as magnesium having a specific gravity of 1.74 (the specific gravity of aluminum: 2.70) and magnesium alloys are known. However, molded articles of magnesium materials have drawbacks in that the articles are more expensive than the articles of aluminum materials and defect articles are produced in a greater amount than that of the articles of aluminum materials. Therefore, a molded article of an inexpensive aluminum material having a weight as light as that of the molded articles of magnesium materials is desired.
To produce a molded article of an inexpensive aluminum material having a weight as light as that of the molded articles of magnesium materials, it is necessary that the thickness of the article be reduced. However, as described above, it is difficult that a die-cast molded article of an aluminum material having a thickness smaller than 1 mm is produced in accordance with the conventional technology. Therefore, development of technology for efficiently producing a die-cast molded article of an aluminum material having a thickness smaller than 1 mm has been desired.
The present inventor studied the process for producing a die-cast molded article of an aluminum material having a thickness smaller than 1 mm and it was found that a die-cast molded article of an aluminum material having a thickness smaller than 1 mm can be obtained when one or more porous iron members which do not allow the melted metal of the aluminum material to pass but allow gases to pass and are connected to a passage of gases open to an outside are disposed in portions of members surrounding the cavity of the mold, a cavity of a mold is filled with a melted metal of the aluminum material under pressure and gases and the air are released to the outside through the porous members.
When the melted metal of an aluminum material is introduced into the mold under pressure to fill the cavity, the aluminum material is introduced through a gate. The aluminum material at the gate portion is separated from the obtained die-cast molded article after solidification. Therefore, the thinner the gate, the smaller the defect portion formed on the molded article and the better the appearance of the molded article. However, since an excessively thin gate causes poor flow of the melted metal, in general, a gate having an opening 0.6 to 1 times as much as the opening of the die-cast molded article has heretofore been used in conventional processes for producing a die-cast molded article of an aluminum material having a thickness of 1 mm or greater. In other words, the maximum  minimum opening of the gate is about 0.6 mm. When the opening of the gate is smaller than this value, the flow of the melted material is insufficient and a die-cast molded article of an aluminum material having excellent quality cannot be obtained.
A gate which provides excellent flow of the melted metal even when the opening of the gate is about ½ of the thickness of the molded article has been desired in the process for producing a die-cast molded article of an aluminum material having a thickness smaller than 1 mm. It is expected that a thin die-cast molded articles of an aluminum material exhibiting excellent appearance and having excellent quality can be obtained by using a thin gate providing excellent flow of the melted metal.